This invention relates to archery bows and, in particular, to an archery bow having molded limbs including a pre-formed reinforcing sheet.
An archery bow is basically a two armed spring having a grip at the middle and held in a flexed or bent position by a string connected to each end of the spring. Energy stored in the bow as it is drawn is transferred to the arrow when the bow is fired. Despite the conceptual simplicity of a bow, actually making a durable, consistent bow has been the work of skilled craftsmen for millennia and continues today.
The simplest bow is made from a single material, typically yew, which is a fine grained, Old World wood used for making cabinets and bows. Even this simple bow requires careful shaping of the yew shaft to control curvature and draw force. Early composite or laminated bows of wood, horn, and sinew provided greater power and durability and permitted the maker to "recurve" the limbs, i.e. to curve the ends of the limbs away from the archer. A recurve bow can be made relatively short from end to end, yet have a long draw, that is, a large distance from the grip on the handle to the nock of the arrow at full draw. A recurve bow also exhibits some "letoff" or reduction in draw force (known as draw weight or simply weight) at full draw, depending upon the motion of the "ears" or the free ends of the limbs. These characteristics of a recurve bow are obtained to an even greater degree in a "compound" bow, in which pulleys replace the ears.
A bow having laminated limbs and a cast metal handle is disclosed in U.S. Pat. No. 3,659,577 (Richardson et al.) The limbs are described as a combination of glass fiber outer layers and wooden veneer inner layers. The limbs are permanently attached to the handle with epoxy adhesive and pins. Although laminated bows made entirely from wood are still made today, a modern bow is typically made in three sections: a central handle or riser and two separate limbs. The handle is typically made from machined aluminum or magnesium. Some limbs are machined from a glass/epoxy laminate or laminated from glass and wood. Some limbs are molded from fiberglass reinforced resin, as described in U.S. Pat. No. 4,735,667 (Johnston).
Bow limbs must withstand large forces resulting from drawing and firing a bow. The problem is more acute in a compound bow in which pulleys are attached to the free ends of the limbs and laced with cable to give an archer a mechanical advantage in drawing the bow. When the pulleys are mounted eccentrically, the pulleys increase the effective length of the limbs at full draw, reducing the required draw force. A reduced draw force at full draw permits the peak draw force of the bow to be increased even more.
As used herein, "lacing" refers either to a one piece bowstring or to a three piece line including two end cables connected by a central stretch between the pulleys which forms the bowstring. The cables and bowstring are not attached to a single point at the end of a bow limb but are spaced across the width of the end of the limb. The substantial forces from the lacing combined with the spacing cause torques on the free ends of the limbs which twist the limbs. The torques vary as the bowstring is drawn and released. In general, the tension on the bowstring is greatest when a bow is at rest and is least when the bow is fully drawn. In the cables, tension is least when the bow is at rest and is greatest when the bow is drawn. As a result of these changing forces, the ends of the limbs twist one way and then the other each time that the bow is drawn and fired. The sideward force from using a finger release and the sideward force from a cable guard pushing the cables to one side also contribute to twisting the limbs as a bow is drawn and fired.
Bows can be made in any weight (peak draw force) that a customer may wish. Limbs of different weights can be made from a single mold by changing the thickness of the limbs. Stiffer limbs are thicker, and somewhat heavier, than limbs that are more easily flexed. The ability of a limb to resist twisting depends primarily upon the stiffness of the limb.
It is desired to separate these two characteristics without unduly complicating the process for making limbs. In particular, it is desired to make a limb that resists twisting but is not so stiff as to produce a bow having a high draw weight. It is also desired to control the stiffness of a limb without increasing the thickness or weight (mass) of the limb.
Custom limbs can be made by producing each limb by its own, unique process. Custom manufacturing using a plurality of different processes and equipment is very costly and impractical. What is desired is the ability to produce bow limbs having wide variety of characteristics, as if the limbs had been made by custom processes, but by using essentially the same process and equipment for all the different types of limbs.
In view of the foregoing, it is therefore an object of the invention to minimize twisting in the tips of the limbs of an archery bow.
Another object of the invention is to make bow limbs with a variety of combinations of characteristics from the same, basic process.
A further object of the invention is to provide a bow limb that has the same stiffness but less mass than a bow limb of the prior art.
Another object of the invention is to control the motion of a bow limb with a pre-formed insert in the limb.